Excimer gas lasers remain the principal laser system of choice for refractive eye surgery by photo-ablation, in which corneal eye issue is vaporised while causing little or no thermal damage to adjacent areas. Notwithstanding their widespread use, excimer lasers have a number of inherent disadvantages, including large size and high operating and maintenance costs, and reliance on an extremely toxic gas that must be regularly replaced. Excimer lasers have an operating wavelength of 193 nm, in the ultraviolet region of the electromagnetic spectrum.
Alternative, solid state laser systems have been proposed for generating an ultraviolet laser beam, suitable for corneal ablation, by frequency conversion of the output of an infra-red solid state laser, such as the widely used Neodymium:YAG laser. The Nd:YAG laser produces a wavelength of 1064 nm, and this output beam is directed through a sequence of non-linear optical (NLO) crystals to derive an appropriate harmonic in the ultraviolet region by a process known as harmonic generation. Such systems are generally described in, eg U.S. Pat. Nos. 5,144,630 and 5,592,325. International patent publication WO 99/04317 discloses a solid state laser system in which an Nd:YAG laser beam is passed in sequence through a crystal of bet barium borate (B—BaB2O4 or BBO) and a pair of crystals of caesium lithium borate (CsLiB6O10 or CLBO) to generate the fifth harmonic of the Nd:YAG laser output at 213 nm, which has been found to be especially suitable for refractive surgery by photo-ablation. This harmonic ha also been produced using three BBO crystals (Lago et, 1988, Optics Letters 13 (3): 221–223).
The aforementioned International patent publication WO 99/04317 and related U.S. continuation-in-part application Ser. No. 09/484777 disclose arrangements for mounting the frequency conversion crystals in hermetically sealed housings with in-built heater elements for maintaining the crystals at optimum temperatures, which is important for the stability of the frequency conversion process. In one embodiment, the two CLBO crystals are mounted together in optical contact in the one housing, while in the other they are mounted in separate housings. The present applicant has found that solid state laser systems arranged in this way have produced a satisfactory 213 nm ultraviolet laser beam suitable for photo-ablation of biological tissues such as corneal tissue. However, improvement in stability and uniformity of the beam would be advantageous. There has also been an observed degradation of the crystals at a rate which would not be acceptable in a practical commercial system.
To address small fluctuations in the direction of the laser beam as it emerges from the crystal, European patent publication 1 041 427 discloses a crystal holder fitted with “beam passage components” adjacent the incident and exit faces of the CLBO crystal to reduce localised air shimmer arising from the crystal heating system. In another arrangement disclosed in European patent publication 1 048 974, which is also concerned with reducing crystal interface degradation, elongate hermetically sealed spaces extend from the respective crystal housing windows, and these spaces are filled with high purity oxygen or a gas mixture of high purity oxygen and an inert gas.
This invention is primarily directed to the provision of solid state laser systems of enhanced beam stability and uniformity, and with a longer crystal life than presently observed.